1. Field of the Invention
Rheology is defined as the science of the flow and deformation of matter. Some materials approach the behavior of ideal fluids and are described as being viscous. Other materials approach the behavior of ideal solids and are described as being elastic. Most materials exhibit a combination of viscous and elastic behavior and are described as being viscoelastic. The viscoelastic behavior of materials is important in both the processing of materials and application of materials.
For example, a polymer material to be processed is put into a fluid state either by the application of heat or by dissolving it in a low molecular weight solvent to form a solution. The resulting fluid is of a viscoelastic nature and may be formed into a fabricated shape either by flow under the force of gravity or by flow under an externally applied pressure. In order to properly design such processing operations and the equipment associated therewith it is important to have accurate measurements of the rheological behavior of the viscoelastic polymer melt or solution.
It is also important to have accurate measurements of the rheological properties of materials other than polymers such as inks, colloidal suspensions, copying toners, etc. The usefulness of these materials is greatly dependent on their rheological characteristics. Customer acceptance of foods such as mayonaise, puddings, and bread doughs are dependent on their rheology, as well as tooth pastes and cosmetics.
2. Description of the Prior Art
There is a large amount of prior art dealing with rotational rheometers. These take many forms. For example, U.S. Pat. No. 3,680,366 discloses a torsional operating device which does not measure the recoverable strain of the material tested, an important objective of the present invention. Similarly Masashi Umeno et al in U.S. Pat. No. 3,479,858 discloses a torsional oscillating device for measuring viscoelasticity but this system does not measure elasticity or recoverable strain, an important feature of the present invention.
U.S. Pat. No. 2,484,761 shows strain gauges attached to an arm to measure stress. Although this system might be used with the apparatus based on the present invention it does not measure recoverable strain.
Maxwell in U.S. Pat. No. 4,092,849 discloses an apparatus very similar in design and objective to the present invention but the bearing system therein is made up of conventional ball bearings which have the undesirable features of being difficult to align, as well as being higher than desired in friction and requiring constant cleaning to produce reliable results. The objective of the present invention is to overcome these undesirable characteristics by providing an improved bearing system that insures alignment and minimal friction.
U.S. Pat. No. 4,566,324 describes a rotational rheometer of quite conventional geometry with an elaborate radiation double refraction system for measuring specimen deformation. The present invention does not involve any such system. This apparatus does not measure recoverable strain.
Garritano in U.S. Pat. No. 4,601,195 discloses a device for measuring the viscoelastic properties of materials which is perported to measure both the viscous and elastic properties. The elastic properties are assumed to be proportional to the normal force or "Weissenberg Effect". The present invention does not use this or any other assumption but rather measures the elastic properties directly by measuring the recoverable strain. The Garritano apparatus does not do this. The two systems operate on entirely different principles.
Myerholtz in U.S. Pat. No. 3,277,700 presents an improved torsional pendulum. This produces an oscillatory signal to indicate the properties of the specimen. This principal is not used in the present invention.
Zemp et al in U.S. Pat. No. 3,545,257 describes a rotational viscometer for measuring the viscosity of materials in which a hydrostatic support means is used to provide a low friction bearing. This is not the system of the present invention. The Zemp apparatus does not attempt to measure elasticity of recoverable strain.
SU Patent No. 1111072-A presents a rotating viscometer with bearing means consisting of pins protruding axially from both ends of the central cylindrical member. This is exactly the opposite to the present invention. If these pins of this patent become bent, then the control cylindrical member will not rotate about its axis of symmetry but rather will wobble. The apparatus of SU No. 1111072-A does not measure elasticity or recoverable strain.
WO Patent No. 86/00408 describes a rotation viscometer with an electromagnetic system for detection of the motion of the inner cylindrical member. The upper bearing consists of a pin protruding axially from the rotating member. This is exactly the opposite to the present invention and suffers from the same undesirable characteristics as SU No. 1111072-A.
SU Patent No. 1062-565-A describes a normal stress meter in which the bearing system consists of pins protruding axially inward from the frame to ball bearings in the ends of the rotating member. This patent differs particularly from the present invention since the ball bearings are higher in friction than the system presently disclosed. Also ball bearings need lubrication and cleaning. The system of the present invention does not.
Finally since the test measurements are made at various temperatures the system of SU No. 1062-565-A has a very serious defect. With an increase in temperature the rotating member expands axially. This produces an increasingly strong axial force on the bearing due to the tapered pin and ball bearing configuration. This produces a large increase in friction and it may even be possible to freeze the bearings with this system. The present invention overcomes this problem by allowing the rotating member to expand freely.
The current apparatus for measuring rheological properties have been reviewed in detail in (1) Rheometers for Molten Plastics by John M. Dealey, published by Van Nostrand Reinhold Co. The final paragraph of this book points out that a need definitely exists for an inexpensive rheometer capable of measuring rheological characteristics including elasticity. It is an important object of the present invention to satisfy this need.
Past efforts in devising apparatus for the measurement of rheological properties have resulted in complex and expensive apparatus requiring extensive data reduction.
One attempt to devise a rheometer to measure elastic recoverable strain is disclosed in U.S. Pat. No. 4,092,849 granted to the present inventor. This apparatus is marketed under the name of Melt Elasticity Tester and is discussed by Dealey in the above reference. He describes the principal of operation as simple and straight forward but mentions difficulty in loading the specimen in the apparatus. He also describes the bearings used to hold the rotatable members coaxially aligned as requiring careful attention. It is an important objective of the present invention to overcome this problem.
It is well known that any rheometer used to measure the specimens elastic recoverable strain characteristics must be as close to friction free as is physically possible. This has led in the past to the use of air bearings to support the rotatable members, but it has been found that all such bearings include residual torque, which increases the difficulty in interpreting the data. Also, air bearings are expensive to manufacture and require a supply of ultra pure air for operation.
The present invention overcomes many of the undesirable features of previous rheometers due to the simple construction of the almost friction free coaxially aligning bearings.